Many machines require a power supply to convert incoming AC voltage (for instance from the mains) to low voltage DC as required by circuitry within the machines. One method of achieving this is the use of linear power supplies. These are relatively uncomplicated, and employ a mains transformer, rectifiers, smoothing capacitors, power semiconductor pass elements and small active/passive feedback components to stabilise the low voltage DC. The primary drawback of linear power supplies is that they are heavy, bulky and only around 40% efficient, which gives rise to a lack of competitiveness.
An alternative is to use a switched mode power supply (SMPS). A SMPS connects the incoming AC power supply to the load (ie the machine to be powered) by a forward-biased diode bridge and comprises a bulk capacitor connected in parallel with the load. A schematic representation of the circuitry of a rectifier stage of a basic SMPS is shown in FIG. 1 of the accompanying drawings.
SMPS's are, in general, more efficient than linear power supplies, and 70%–80% efficiency at full rated load is readily achievable. The size of the energy storage components can also be much less, due to the high switching frequency compared with mains input. These advantages make SMPS's a favourable option. SMPS's presently comprise around 60% of the power supplies manufactured worldwide.
One drawback of both SMPS's and linear power supplies is that these devices draw an inherently non-sinusoidal current from AC power sources. This is due (in the case of SMPS's) to the fact that, since the bulk capacitor and the power source are connected to one another by a forward-biased diode bridge, current will only flow from the power source to the bulk capacitor and the load when the power supply voltage exceeds the voltage across the capacitor. No current will flow from the power source at other times. Clearly, this leads to short periods of current flow near the peak of each AC cycle of the power source. The effect of this is to introduce undesirable harmonics into the power source.
The introduction of harmonics has a number of undesirable impacts on the electrical distribution system including increased root mean square (ie heating) current in the system wiring for a given load. This results in a reduced power factor of the electrical current drawn from the AC power source and may cause tripping of protection equipment at lower power delivery levels than would otherwise be the case.
At the time of writing, new regulations are to be introduced that set a limit on the harmonics associated with the current distortion described above. It will soon be mandatory for the harmonic levels introduced by a power supply to be within the limits set by the regulatory specifications. One approach to complying with these regulations, when using an SMPS, is the use of passive power factor correction, using an inductor, with little or no additional circuitry, to draw a smoother current from the power source.
Passive power factor correction requires relatively few components, and in its simplest form comprises an inductor located at any point in the rectifier circuitry, provided that it is placed before the capacitor. The inductor is often located between the forward-biased diode bridge and the bulk capacitor, for reasons that will be explained below. The competitive nature of, in particular, the market for personal computer power supplies (for which SMPS's are well-suited) generates great pressures to minimise costs. For this reason, the simplicity of design offered by passive power factor correction is an attractive feature. However, the size and weight of the inductor introduced into the power supply is a key consideration.
In order to comply with present harmonic current legislation, any device drawing an input power greater than 50W must limit the current harmonics introduced into the power source to within specified levels, which are dependent on the power drawn. It is, for a device that may draw an input power above 50W, necessary to provide an inductor that will maintain the introduced current harmonics to below the specified levels when the device draws an input power between 50W and full input power. If there is a significant power range over which compliance with harmonic regulations is to be achieved, an inductor whose inductance varies with the current flowing therethrough is essential if the size and weight of the inductor are to be kept to a minimum.
In modern inductor design, in order to maximise the energy associated with the flux in the core of an inductor, and therefore to reduce the size of the inductor, it is normal to introduce a small air gap into the magnetic circuit comprising the inductor. This can, in certain types of core, be achieved by the introduction into the magnetic circuit of a thin piece of insulating material of the thickness required, to maintain the correct dimensions of the “air” gap. As saturation of the core is reached, the relative permeability of the core will tend towards unity, equaling the permeability of the air gap. The presence of such an air gap leads to an inductor having an inductance that varies with the current passing therethrough. The provision of a core having a profiled air gap (i.e. one having a varying width) allows control to be exercised over the variation of the inductance with current, and this phenomenon may be exploited to produce an efficient inductor for passive power factor correction, as described above.
However, the behaviour of such an inductor is extremely difficult to model, and a drawback of this technique is that it is very difficult to predict the inductance-current relationship of a stepped-gap inductor without actually building one.